Resistive devices can be used as semiconductor resistors, switches, or memory elements (e.g., memory cells of a memory device), among other applications. Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), flash memory, and resistive random access memory (RRAM), among others.
Memory devices are utilized as non-volatile memory for a wide range of electronic applications in need of high memory densities, high reliability, and low power consumption. Non-volatile memory may be used in a personal computer, a portable memory stick, a solid state drive (SSD), a personal digital assistant (PDA), a digital camera, a cellular telephone, a portable music player (e.g., MP3 player), a movie player, and other electronic devices, among others. Program code and system data, such as a basic input/output system (BIOS), are typically stored in non-volatile memory devices.
Memory cells can be arranged in a matrix (e.g., an array). For example, an access device (e.g., transistor) of a number of memory cells may be coupled to an access line (one example of which is a “word line”) forming a “row” of the array. The memory elements of each memory cell are coupled to a data line (one example of which is a “bit line”) in a “column” of the array. In this manner, the access device of a memory cell is accessed through a row decoder activating a row of memory cells by selecting the word line coupled to their gates. The programmed state of a row of selected memory cells is determined by causing different currents to flow in the memory elements depending on the resistance associated with a programmed state for a particular memory cell.
Memory cells can be programmed (e.g., write, erase) to a desired state. That is, one of a number of programmed (e.g., resistance) states can be set for a memory cell. For example, a single level cell (SLC) can represent one of two logic states (e.g., 1, 0). Resistive memory cells can also be programmed to one of more than two programmed states, such as to represent more than two binary digits (e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, 1110). Such cells may be referred to as multi state memory cells, multi-digit cells, or multilevel cells (MLCs).
Non-volatile resistive memory such as RRAM stores data by varying the resistance of a resistive memory element. Data may be written to a selected memory cell in an RRAM by applying a predetermined voltage, at a predetermined polarity, for a predetermined duration, to a particular resistive element. RRAM can be programmed to a number of resistance states by application of voltage of various magnitudes, polarities, and/or durations.
One type of resistive memory element is a memristor. Memristors can be used to form RRAM. Such an RRAM can be formed of a material that can be configured to provide variable resistance, such as an oxide (e.g., metal oxide such as a transition metal oxide (TMO), nitrides, etc.). The RRAM may utilize a resistance transition characteristic of the TMO by which resistance of the material varies according to a change in application of voltage. Memristors can be implemented in nanoscale devices, thereby enabling storage elements to provide a high density, low cost, non-volatile, high speed RAM without the read/write cycle endurance limitations of charge-storage type memory.
A resistive device can have an active region that is formed of one or more materials that are electronically semiconducting (e.g., nominally electronically insulating) and also are weakly ionic conductor(s). Material(s) of the active region can be capable of hosting and transporting ions that act as dopants to control the flow of electrons through the material(s). Ionic transport may also be understood as the transport of the absence of a particular ion (e.g., ionic vacancies), similar to understanding electric current by the movement of “holes” representing the absence of an electron. That is, ionic vacancies appear to move in a direction opposite to that of the corresponding ions.
According to one previous approach, the active region of a resistive device is formed by depositing two discrete materials that differ in some initial characteristic (e.g., concentration of ionic vacancies). Operation of the resistive device involves transport of ionic vacancies from the first region, across a boundary between the two discrete regions, to the material of the second region. The active region thus comprises, for example, a primary material for transporting and hosting ions that act as dopants to control the flow of electrons, and a secondary material for providing a source of ionic dopants for the primary material. However, the physical boundary between the two regions of material that differ in some initial characteristic can result in some undesirable consequences.
One limitation of one or more previous resistive device fabrication approaches is an inability to control small changes in atomic and vacancy arrangement during the film stack creation, and to not damage the thin film stack during patterning. Previous methods for creating oxides from metals also tend to be grain boundary sensitive. Grain boundaries can result from a lack of fabrication control while forming a plurality of discrete regions (e.g., materials). Due to small feature size limitations (e.g., sub 20 nm), direct deposition methods are limited.
According to another previous approach, the active region of a resistive device is formed by depositing one material (e.g., a material), and using a voltage (e.g., 5-10 V) in forming the resistive device that is stronger than the electric field (e.g., 2-2.5 V) that is used to thereafter operate the resistive device to form two regions within the active region that differ in some characteristic (e.g., concentration of ionic vacancies). The two regions can form a gradient, with one end of the gradient acting as a first “region” and an opposite end of the gradient acting as a second “region.” However, the application of a strong electric field, perhaps for an extended period of time, in initially forming the two regions with one material can result in some undesirable consequences. For example, application of a voltage large enough to initially form the two regions (e.g., 5-10 V) can cause dielectric leakage and/or change the threshold of the dielectric.
Furthermore, applying a higher voltage bias to resistance device causes both electron current and ion current to flow, whereas at a lower voltage bias the flow of ion current is negligible, which allows the switch to hold its resistance state. Therefore, where a strong electrical field is used initially in forming two regions, the higher voltage bias can cause unintended ions in the molecular structure to move (e.g., sodium ions can become mobile and move as the oxygen vacancies are being moved). For these and other reasons, a method for forming two regions (e.g., portions) of an active region, the regions being defined based on ionic species concentration (e.g., of oxygen vacancies), without having to move the ionic species using relatively strong electric fields, would be advantageous.
One previous mode of operating the resistive device after formation is to apply a relatively large electrical field across the resistive device that exceeds some threshold for enabling the motion of the ions (or vacancies) in the material(s) to cause an ionic species (or vacancy thereof) to be transported via ionic transport (e.g., into or out of the first material from/to the second material, or into or out of a first region from/to a second region initially formed of similar material and subsequently altered by application of a strong electric field).
Accordingly, a method for forming a resistive device without the disadvantages associated with the deposition of two separate materials (e.g., grain boundaries) or the application of a strong electrical field for initially in forming two regions would be beneficial.